Recently, in a backbone DWDM (Dense Wavelength Division Multiplex) optical communication system, a relay optical amplifier is used on an optical transmission line. Due to limitations of the transient response characteristics of the optical amplifier, it is generally preferable that an optical transmitter as an optical input source to the optical amplifier has a specification to gradually increase an optical output at the time of activation over a certain period of time.
FIG. 1 is a diagram illustrating an example of an optical output rise specification requested for an optical transmitter.
As illustrated in the upper part of FIG. 1, when a setting signal for an optical source of the optical transmitter is changed from “OFF” to “ON,” as illustrated in the lower part of FIG. 1, in an optical output equal to or greater than Pmin, the optical output gradually rises at a slope (0 to 1 mW/sec) within a specification range. Here, in the case of an optical output equal to or less than Pmin, an optical amplifier on a transmission line is not activated, and therefore control may be not done in the case of an optical output equal to or less than Pmin. In the case of an optical output greater than Pmin, the optical amplifier starts to be activated, and therefore it is controlled such that the light output gradually increases.
FIG. 2 is a diagram for explaining content of control performed while an optical output gradually increases.
In the related art, by controlling an injection current to an LD (Laser Diode) of an optical transmitter and linearly increasing an LD optical output to, for example, a defined value over a certain period of time, a slope of the optical output rise is made gentle. Also, for example, as illustrated in Japanese Laid-open Patent Publication No. 2008-092172, in a case where an optical transmitter is configured with an LD and a phase modulator, a bias voltage or amplitude of an electric signal to drive the phase modulator is controlled (i.e. ABC (Auto Bias Control) control) to an optimum value. The control starts when an optical input (i.e. LD optical output) to the phase modulator reaches Pmin. Here, in the ABC control, pilot frequency of the lower frequency is superimposed over an optical signal and the control is performed based on a detection value of this component.
In a case where the phase modulator is configured with a plurality of stages, the ABC control of each stage is performed in a general time-sharing manner. Here, ABC control in the case of a DP-QPSK (Dual Polarization Quadrature Phase Shift Keying) modulation system will be described. In the case of performing ABC control of the multiple stages in a time-sharing manner, for example, as illustrated in FIG. 2, six adjustment periods corresponding to intervals I to VI are set. In FIG. 2, a time interval of twelve seconds is divided into six adjustment periods and different ABC control is performed on each adjustment period. Subsequently, these six adjustment periods are repeatedly set until an LD optical output reaches a defined value. That is, control supporting the six adjustment periods is repeatedly performed.
To be more specific, in interval I, bias control of π/2 shift on an X-side QPSK modulator is performed. In interval II, control on the modulation amplitude of an I signal and bias control on the I signal are performed in the X-side QPSK modulator. In interval III, control on the modulation amplitude of a Q signal and bias control on the Q signal are performed in the X-side QPSK modulator. In interval IV, bias control of π/2 shift on a Y-side QPSK modulator is performed. In interval V, control on the modulation amplitude of an I signal and bias control on the signal are performed in the Y-side QPSK modulator. In interval VI, control on the modulation amplitude of a Q signal and bias control on the Q signal are performed in the Y-side QPSK modulator. Here, as an example, each adjustment period is two seconds.
Also, in the bias control and the amplitude control in intervals II, III, V and VI, two different kinds of pilot frequencies are used. To be more specific, two kinds of pilot frequencies superimposed over optical signals are detected, and the bias control and the amplitude control are performed based on the detected values.
Thus, in the related art, control of a drive signal amplitude, DC bias and phase shifter is performed in a DQPSK/QPSK optical phase modulator.
Meanwhile, in ABC control in a phase modulator in the related art, the following variation occurs immediately after the start of control (especially control of a bias voltage and amplitude).
a) The ABC control largely moves toward a convergence point (i.e. optimum point) and therefore an optical power loss in the phase modulator largely varies.
b) The phase modulator and its drive circuit are generally mounted in neighbor positions because of high frequency characteristics. Therefore, when an output amplitude of the drive circuit increases, power consumption of the drive circuit increases and heat generation increases, and temperature increase is caused in the phase modulator due to this heat generation. Due to this influence, light transmission characteristics (i.e. applied voltage vs. optical power loss) of the phase modulator shift in a short period of time, and therefore an optical power loss in the phase modulator largely varies in an interval in which bias control is not performed (i.e. an interval in which a voltage is held).
FIG. 3 and FIG. 4 are diagrams for explaining a phenomenon where, when ABC control largely moves toward a convergence point (i.e. optimum point), an optical power loss largely varies in the phase modulator.
FIG. 3 is a graph in which the horizontal axis represents an applied voltage of the phase modulator (i.e. a modulation voltage applied to a modulation electrode of the phase modulator, which can be referred to as “drive voltage”) and the vertical axis represents an optical power loss of the phase modulator (i.e. an optical output of the optical transmitter). The modulation voltage or the drive voltage is controlled by a modulation amplitude at which a voltage for modulation changes and a bias voltage which is the main voltage of the change.
Here, the optical power loss of the phase modulator (i.e. an optical output of the optical transmitter) represents changes in the optical output at the time of a change of the applied voltage held by the phase modulator in a case where an LD optical output is maximum.
FIG. 3 illustrates an optical power loss characteristic of the phase modulator with a sine curve characteristic and further illustrates a state of an applied voltage below the horizontal axis. The central point of the amplitude of an applied voltage is referred to as “bias point.” The bias point may largely vary depending on ABC control. In FIG. 3, the bias point is represented by black circles. In the case of the phase modulator, the optimum position of the bias point is a point at which the optical output is minimum.
In FIG. 3, when an applied potential moves from 1 to 2, the optical output is indicated by 3 when the applied voltage is 1, and the optical output is indicated by 4 when the applied voltage is 2. As seen from comparison between the optical outputs 3 and 4, the optical output power loss (i.e. optical output) largely varies.
FIG. 4 is a graph where the horizontal axis represents an applied voltage of the phase modulator and the vertical axis represents an optical power loss of the phase modulator (i.e. an optical output of the optical transmitter). FIG. 4 illustrates a case where the applied voltage is originally 1 and the amplitude of the applied voltage rapidly increases by ABC control as illustrated in 2. The optical output is 3 when the applied voltage is 1, and the optical output is 4 when the applied voltage is 2. As seen from comparison between the optical outputs 3 and 4, the optical output largely varies.
FIG. 5 is a diagram for explaining a phenomenon where an optical power loss in the phase modulator largely varies when light transmission characteristics of the phase modulator shift by temperature increase. FIG. 5 is a graph where the vertical axis represents an applied voltage of the phase modulator and the vertical axis represents an optical power loss of the phase modulator (i.e. an optical output of the optical transmitter). In FIG. 5, light transmission characteristics of the phase modulator as illustrated by a sine curve shift to the right side by rapid temperature increase. Although the applied voltage does not vary, it is found that the optical output varies from 1 to 2.
FIG. 6 is a diagram for explaining a problem in the related art.
As a result of above (a) and (b), a case occurs where the optical output rise slope of the optical transmitter becomes rapid or negative as illustrated in FIG. 6, and therefore the optical output does not rise at a slope within a specification range. Here, FIG. 6 is a pattern diagram and does not definitely illustrate that the slope becomes rapid or negative in intervals II, III, V and VI as illustrated in FIG. 6. However, by performing ABC control in intervals I to VI, there can occur a case where, in any of these intervals, the optical output becomes rapid or negative.
With this phenomenon, the optical input level with respect to an optical amplifier, which is set as a repeater on a transmission line of a DWDM optical communication system, changes rapidly or increases and decreases (i.e. ringing). The response time of gain control (i.e. AGC or Automatic Gain Control) in the optical amplifier is slow, and therefore it is difficult to respond to the rapid change or ringing of this optical input level and the optical output level from the optical amplifier transiently changes. The variation of this optical output level causes degradation of transmission quality.
FIG. 7 is a diagram for explaining a state where transmission quality degradation is caused in the DWDM optical communication system.
It is assumed that the optical signals of respective wavelengths are output from optical transmitters 1 to N and a rapid change or ringing occurs in the optical output from the optical transmitter 1. The optical signals of respective wavelengths from the optical transmitters 1 to N are wavelength-multiplexed in an MUX 30 and transmitted. The wavelength-multiplexed optical signal wavelength-multiplexed in the MUX 30 is amplified as is in an optical amplifier 31. Here, since a rapid change or ringing occurs in the optical output from the optical transmitter 1, a rapid change or ringing occurs in the whole wavelength-multiplexed optical signal too. However, since the response time of AGC control in the optical amplifier 31 is slow, it is difficult to respond to the above, and the optical output from the optical amplifier 31 has a change different from the rapid change or ringing. If the respond is possible, the other optical signals of wavelengths than the optical signal of the wave length with the ringing are kept constant. However, since the respond is not possible, an optical output change also occurs in the other optical signals of wavelengths.
Therefore, such a wavelength-multiplexed optical signal is wavelength-demultiplexed in a DeMUX 32, and, when another group of optical transmitters 1 to N receive the results, an optical output change occurs in the optical signals of all wavelengths. This optical output change causes degradation of transmission quality on the receiving side.